1. Field of the Invention
The present invention relates to a vibration proof optical system and, in particular, relates to a telephoto lens to be mounted on a film camera or an electronic camera using an imaging device such as a CCD (charge coupled device) and relates to an imaging apparatus which mounts the telephoto lens as an imaging lens system.
2. Description of Related Art
There is known a method in which a part of lenses within an optical system is moved in a direction orthogonal to an optical axis in order to correct the image vibration of a taken image, caused by the vibration of the optical system due to hand movement. Japanese Patent No. 3590845 discloses the invention of an image position correcting optical system which includes a first lens group G1 having a positive refractive power, a second lens group G2 having a negative refractive power and a third lens group G3 having a positive refractive power in the order from the object side, wherein the second lens group G2 is moved along the optical axis to perform the focusing and the image correction group G3S of the third lens group G3 is moved in the direction orthogonal to the optical axis to correct the image position.
In a telephoto lens, chromatic aberration degrades as the focal length becomes longer and the optical system is made smaller. Further, the weight of the telephoto lens increases as the focal length becomes longer and so the image vibration due to the hand movement will be caused at the time of imaging. However, when a vibration proof mechanism for correcting the image vibration due to hand movement is employed, the lenses become larger. The vibration proof optical system of the related art requires lenses of a relatively large diameter so that the vibration proof lens group does not shade a light flux when the vibration proof lens group moves vertically with respect to the optical axis at the time of absorbing the vibration. Further, at the time of absorbing the vibration, since the lens system becomes nonrotational symmetry, there may arise such phenomena that the chromatic aberration etc. appear at the center portion of a screen and an image plane falls down at the periphery of a screen. To this end, there has been desired to develop a vibration proof optical system which is less in the degradation degree of performance at the time of absorbing the vibration while suppressing the diameter of the vibration proof lens group to the minimum value.
In the optical system described in Japanese Patent No. 3590845, a ratio between the moving amount of the vibration proof lens group and the moving amount of an image at an imaging plane is almost 1. However, in the case of a longer focal length, the vibration proof effects can be attained more effectively with a small moving amount when the ratio of the moving amount of an image with respect to the moving amount of the vibration proof lens group is set to be larger than 1. Further, the vibration proof mechanism can be further miniaturized when the moving amount at the time of the vibration proof is smaller. Furthermore, the patent document 1 discloses only one embodiment in which the entirety of the third lens group G3 is set as the image correction group G3S. Japanese Patent No. 3590845 does not concretely disclose as to a case where a part of the lenses of the third lens group G3 is set as the image correction group G3S. That is, Japanese Patent No. 3590845 does not explain concretely as to how a part of the lenses of the third lens group G3 is to be optimized as the image correction group G3S.